There are several known methods and apparatuses for pumping high consistency liquid-fiber suspensions such as pulp. Previously only displacement pumps, such as screw pumps or the like, were used to pump high consistency pulp. Nowadays the tendency has been to replace the displacement pumps because of their deficiencies and other inherent drawbacks. The intention has been to develop a centrifugal pump, normally used for pumping water and the like, for pumping high consistency pulp. One of the first problems encountered when attempting to pump liquid-fiber suspensions having a consistency of more than 8% is that the suspension does not independently flow to the impeller of the pump in the suction opening. It has been suggested as a solution to this problem that a special screw feeder be used to feed the suspension to the suction opening of the pump or that a so-called inducer be arranged in the suction opening of the pump to convey the suspension in the suction opening towards the pump impeller. However, with this type of pump it has not been possible in commercial practice to pump fiber suspensions having a consistency which approaches 15%. A third example of possible solutions is a so-called fluidizing centrifugal pump used for pumping high consistency pulp, in which pump the fluidizing rotor extends to the suction opening of the pump or in some cases through it as far as into the mass tower. By using this type of fluidizing rotor it has been possible to reach the above-mentioned consistency of 15%, which does not, however, satisfy all requirements for pulp conveyancing in the pulp and paper industry.
Another possibility is, of course, the dilution of the pulp prior to the pump and the re-thickening of the pulp subsequent to the pump, as has been done with different pulp treating devices according to prior art techniques. This known technique, however, also has disadvantages such as the increased need of technical auxiliary devices, because both feeding and mixing devices are needed in the pulp vessel prior to the pump and thickening devices are needed subsequent to the pump for bringing the pulp back to its original consistency. A further disadvantage resides in the increased consumption of energy, which is required for the thickening of the pumped pulp.